Investment casting is a process for forming metal or metal alloy articles (also referred to as castings) by solidifying molten metal or alloys in molds having an internal cavity in the shape of such articles. The molds are formed by serially applying layers of mold-forming materials to wax patterns formed in the shape of the desired article. The first layer applied to the pattern, referred to as the facecoat, contacts the metal or metal alloy being cast during the casting process. Materials used to form the facecoat, and perhaps other "backup" layers of the mold, can flake off the mold and become embedded in the molten metal or alloy during the casting process. As a result, the metal or alloy article includes a material or materials not intended to be part of the article, such material or materials being referred to as "inclusions".
Many industries, particularly the aerospace industry, have stringent specifications as to the acceptable content and/or size of inclusions. The location of inclusions in castings can be difficult, and in some cases prior to the present invention, impossible to detect. Some inclusions, if detected, can be removed from the metal article, and the article repaired, without compromising its structural integrity.
Titanium has been used by the investment casting industry primarily for casting articles having relatively small cross sections. However, investment casting is now being considered for producing structural components of aircrafts having significantly larger cross sections than articles cast previously. Certain inclusions in relatively thin articles can be detected using X-ray analysis. For example, thorium oxide and tungsten have been used as refractories to produce molds for investment casting. Some thorium oxide and tungsten inclusions could be detected in titanium castings by X-ray analysis because there is a sufficient difference between the density of thorium oxide and tungsten and that of titanium to allow imaging of thorium-oxide or tungsten-derived inclusions. This also generally has proved true of articles having relatively small cross sections cast using molds having yttria facecoats. The difference between the density of yttria and that of titanium is sufficient to allow detection in relatively thin parts, such as engine components. But, X-ray detection cannot be used to image yttria inclusions in titanium or titanium alloy articles as the thickness of articles produced by investment casting increases beyond some threshold thickness that is determined by various factors, primarily the thickness of the cast part, the type of metal or alloy being cast, the size of the inclusion and the material or materials used to form the mold. Inclusions also cannot be detected by X-ray if the difference between the density of the facecoat material and the metal being cast is insufficient or if the size of the inclusion is very small.
Thermal neutron radiography (N-ray) imaging agents have been used in the casting industry prior to the present invention. For example, ASTM (American Society for Testing and Materials) publication No. E 748-95 states that "[c]ontrast agents can help show materials such as ceramic residues in investment-cast turbine blades." ASTM E 748-95, p. 5, beginning at about line 46. This quote refers to the detection of ceramic residues by N-ray on articles having an internal cavity produced by initially solidifying metal about a ceramic core. The ceramic core is removed to form the cavity, and thereafter a solution of gadolinium nitrate is placed in the cavity. The gadolinium nitrate solution remains in the cavity long enough to infiltrate porous ceramic core residues that are on the surface of the article. The residues then can be imaged by N-ray. However, this method does not work for imaging inclusions.